1. Field of the Invention
The present invention relates to a turning behavior state detecting system for a vehicle, which is designed to precisely detect the turning behavior state of the vehicle in order to enhance the control accuracy in carrying out the control of the turning motion of the vehicle.
2. Description of the Related Art
There is conventionally known a turning behavior state detecting system for a vehicle in the control of the motion of the vehicle (e.g., in the traction control, anti-lock brake control and the like). Techniques for detecting the turning behavior state of the vehicle are already disclosed, for example, in Japanese Patent Application Laid-open Nos. 2-70561 and 5-155323, and the like.
In the conventionally known system, the turning behavior state of the vehicle is determined based on a difference between a yaw rate detected by a yaw rate detecting means and a reference yaw rate determined based on a steering angle detected by a steering angle detecting means and a vehicle speed detected by a vehicle speed detecting means. However, the reference yaw rate is determined on the basis of a state in which the friction coefficient of a road surface is high. Therefore, in a state in which the vehicle is traveling on a road surface having a low friction coefficient, the reference yaw rate does not correspond to the situation of the road surface, and the detection of the turning behavior state is imprecise.
Accordingly, it is a first object of the present invention to provide a turning behavior state detecting system for a vehicle, which is designed so that the turning behavior state of the vehicle can be detected precisely even in a state in which the friction coefficient of a road surface is low.
It is a second object of the present invention to provide a turning behavior state detecting system for a vehicle, which is designed so that among the turning behavior states of the vehicle, particularly the over-steered state of the vehicle can be detected precisely even in a state in which the friction coefficient of a road surface is low.
To achieve the above first object, according to a first aspect and feature of the present invention, there is provided a turning behavior state detecting system for a vehicle, comprising a vehicle speed detecting means for detecting a vehicle speed, a wheel slip angle calculating means capable of calculating a front wheel slip angle and a rear wheel slip angle based on a lateral slip angle of a vehicle body, a friction coefficient presuming means for presuming a road surface friction coefficient, and a determining means for determining the turning behavior state of the vehicle by comparing the front wheel slip angle and the rear wheel slip angle calculated by the wheel slip angle calculating means with a front wheel slip angle limit value and a rear wheel slip angle limit value which are determined based on the vehicle speed detected by the vehicle speed detecting means and the friction coefficient presumed by the friction coefficient presuming means.
As used herein, the wheel slip angle is an angle formed by the direction of movement of the wheel with respect to the orientation of the wheel, and the grip force of the wheel to the road surface is varied in accordance with the slip angle and the road surface friction coefficient. On the other hand, the lateral slip angle of the vehicle body is an angle formed by the direction of movement of the vehicle body with respect to the orientation of the vehicle body, and indicates the turning behavior state of the vehicle depending on a variation in grip force of the wheel following variations in the turning movement state of the vehicle and the road surface friction coefficient. On the other hand, the limit value of the wheel slip angle can be previously known in accordance with the vehicle speed and the road surface friction coefficient, and the turning behavior state of the vehicle can be detected by comparing the front and rear wheel slip angles calculated based on the lateral slip angle of the vehicle body with the limit value of the wheel slip angle. Moreover, the wheel slip angle is presumed in accordance with the variation in road surface friction coefficient and hence, the turning behavior state of the vehicle can be detected precisely, with the road surface friction coefficient being reflected thereon.
To achieve the second object, according to a second aspect and feature of the present invention, there is provided a turning behavior state detecting system for a vehicle, comprising a vehicle speed detecting means for detecting a vehicle speed, a vehicle body lateral slip angle presuming means for presuming a lateral slip angle of a vehicle body, a friction coefficient presuming means for presuming a road surface friction coefficient, and a determining means for determining that the vehicle is in an over-steered state, when the vehicle body lateral slip angle determined by the vehicle body lateral slip angle presuming means or the rate of variation in vehicle body lateral slip angle exceeds a limit value determined based on the vehicle speed detected by the vehicle speed detecting means and the friction coefficient presumed by the friction coefficient presuming means, during turning movement of the vehicle.
As used herein, the vehicle body lateral slip angle is an angle formed by the direction of the movement of the vehicle body with respect to the orientation of the vehicle body, and indicates the turning behavior state of the vehicle depending on variations in the motion state of the vehicle and the road surface friction coefficient. On the other hand, a limit value for the vehicle body lateral slip angle can be previously known in accordance with the vehicle speed and the road surface friction coefficient, and when the vehicle body lateral slip angle exceeds the limit value based on the vehicle speed and the road surface friction coefficient during turning movement of the vehicle, it can be determined that the vehicle is in the over-steered state. Thus, the over-steered state can be determined precisely, while reflecting a variation in the road surface friction coefficient. When the rate of variation in the vehicle body lateral slip angle is increased during turning movement of the vehicle, it can be determined that the vehicle approaches the over-steered state. When the rate of variation in the vehicle body lateral slip angle exceeds the limit value based on the vehicle speed and the road surface friction coefficient, it can be determined precisely that the vehicle approaches the over-steered state, while reflecting a variation in road surface friction coefficient.